To date, an electric power steering device that provides a steering assist force to a steering system of a vehicle by controlling the drive of an assist motor according to a steering torque acting on a steering member, such as a steering wheel, is well known (see, for example, Japanese Unexamined Patent Application Publication No. 2012-017094).
The electric power steering device according to Japanese Unexamined Patent Application Publication No. 2012-017094 controls the drive of the assist motor according to the steering torque detected by a steering torque sensor. For this reason, if the steering torque sensor enters an abnormal state, it causes a problem in controlling the drive of the assist motor.
To solve such a problem, an electric power steering device according to, for example, Japanese Unexamined Patent Application Publication No. 2012-254649 obtains, if an abnormal state in a steering torque sensor is detected, a steering axial force from a lateral G detected by a lateral G sensor and a yaw rate detected by a yaw rate sensor, in place of a steering torque detected by the steering torque sensor, and generates, in an assist motor, a target torque based on the steering axial force.
The electric power steering device according to Japanese Unexamined Patent Application Publication No. 2012-254649 can maintain a stable steering operation even if the steering torque sensor enters an abnormal state.
However, the electric power steering device according to Japanese Unexamined Patent Application Publication No. 2012-254649 needs at least a lateral G sensor in order to obtain a steering axial force. Furthermore, if the lateral G sensor enters an abnormal state, it causes a problem in controlling the drive of the assist motor.